Thin film photovoltaic (PV) modules (also referred to as “solar panels”) based on cadmium telluride (CdTe) paired with cadmium sulfide (CdS) as the photo-reactive components are gaining wide acceptance and interest in the industry. CdTe is a semiconductor material having characteristics particularly suited for conversion of solar energy (sunlight) to electricity. For example, CdTe has an energy bandgap of 1.45 eV, which enables it to convert more energy from the solar spectrum as compared to lower bandgap (1.1 eV) semiconductor materials historically used in solar cell applications. Also, CdTe converts energy more efficiently in lower or diffuse light conditions as compared to the lower bandgap materials and, thus, has a longer effective conversion time over the course of a day or in low-light (e.g., cloudy) conditions as compared to other conventional materials. Solar energy systems using CdTe PV modules are generally recognized as the most cost efficient of the commercially available systems in terms of cost per watt of power generated. However, the advantages of CdTe not withstanding, sustainable commercial exploitation and acceptance of solar power as a supplemental or primary source of industrial or residential power depends on the ability to produce efficient PV modules on a large scale and in a cost effective manner.
CdTe is a relatively expensive material, and efficient utilization of this material is a primary cost factor in the production of the PV modules. Regardless of the type of deposition system or process, some degree of the CdTe material will inevitably be “wasted” in that it is not deposited onto the PV module. For example, the material may plate out (i.e., condense) on the processing equipment, including shields, conveyor components, vessels, and the like. Recovery and recycling of this material is a key consideration in the industry. In addition, CdTe (and Cd in general) is considered a hazardous material, and the disposal requirements for components that contain CdTe are quite strict and add significantly to the overall cost of the PV module production. Reduction of the volume of these hazardous material components is another primary consideration.
Various references discuss systems and techniques for removal of Cd from scrap metal in general, and PV modules in particular. For example, U.S. Pat. No. 5,405,588 describes a chemical process for recovery of Cd wherein scrap materials containing the Cd are mixed with an ammonium carbonate solution to form a water-soluble ammine complex, which is then evaporated to form a second mixture of cadmium carbonate. The second mixture is further processed to recover the cadmium in the form of cadmium sulfide. U.S. Pat. No. 5,897,685, U.S. Pat. No. 5,779,877 and U.S. Pat. No. 6,129,779 all relate to chemical methods for recovering metals, such as CdTe, from scrap PV modules. Although these processes may have utility, they involve relatively complicated chemical processes requiring acids and other fluids that are expensive, difficult to work with, and pose their own environmental hazards and disposal issues.
U.S. Pat. No. 5,437,705 describes a process and system for recovering cadmium and nickel from Ni—Cd batteries, wherein the scrap batteries and battery components are heated in a retort oven at an effective temperature and time to vaporize the cadmium. The vaporized cadmium is directed into a condensing chamber, wherein the cadmium is condensed to liquid form and directed into molds. This chamber is an elongated tubular component wherein the temperature is maintained at decreasing levels along the length of the chamber, with the lowest temperature being at the outlet of the chamber. Temperature at the molds is maintained high enough to ensure that the cadmium is stored in the molds in liquid form for a time sufficient to allow ash contaminates to rise to the top of the liquid. This condensation system and process is not suited for CdTe recovery in that the unique characteristics of CdTe do not allow for processing of a liquid condensate.
Accordingly, there exists a need for an improved process and system that are uniquely suited for efficient and clean recovery of CdTe from PV modules or components used in the production of PV modules. The present invention relates to a recovery system and process that serve this purpose.